1. Field of Invention
The invention relates to providing electrical power to a plurality of low voltage electrical loads, and more particularly to a string of LED (light-emitting diode) light having a plurality of lamps wired in series, each lamp having an LED and some or all lamps having an additional one of a Zener diode and a resistor connected in series with the LED, thereby eliminating the provision of any additional resistors externally of the lamp.
2. Description of Related Art
LEDs are renowned for their long life and their ability to resist shock. Also, an LED consumes much less electrical power than fluorescent lamps (i.e., energy saving). Therefore, LED lighting devices are gaining popularity worldwide.
A conventional string of lights including a plurality of LED bulbs arranged electrically in a series circuit is shown in FIGS. 1 and 2. AC 120V is rectified by a full-wave rectifier (not shown) to convert into DC (e.g., DC 120V) to be consumed by the plurality of LED bulbs. Each LED bulb is wired in series with a resistor 1. For example, the LED bulb is a blue LED bulb of 3.2V 0.02A. The number of the blue LED bulbs is 25. The light string has a nominal operating voltage of DC 80V. The light string will be damaged if DC 120V is directly applied thereto without any voltage reduction. Resistance (R) of additional resistor(s) other than the LED bulbs of the light string is equal to 40V (i.e., 120V−80V) divided by 0.02 A according to Ohm's Law. Hence, the resistance (R) is 2,000 Ω. Resistor is an electrical component that may generate much heat when energized. 40V power may be applied to a single resistor of 2,000 Ω inserted in series with one of the LED bulbs. Disadvantageously excessive heat generated by the resistor will burn out the LED bulb, thereby killing the circuit. Hence, it is typical to arrange five resistors 1 of 400 Ω each to connect in series to one LED bulb, i.e., five connected LED bulbs in total. Each resistor 1 has a voltage component of 8V and is adapted to generate much less heat. Therefore, the light string is protected.
Another conventional string of lights including a plurality of LED bulbs arranged electrically in a series circuit is shown in FIGS. 3 and 4. AC 120V is rectified by a full-wave rectifier in a plug (not shown) to convert into DC (e.g., DC 120V) which is to be consumed by the plurality of LED bulbs. A resistor 1 is interconnected to the rectifier output and the adjacent LED bulb (i.e., the load). For example, the LED bulb is a blue LED bulb of 3.0V 0.02 A. The number of the blue LED bulbs is 30. The light string has a nominal operating voltage of DC 90V. The light string will be damaged if DC 120V is directly applied thereto without any voltage reduction. Resistance (R) of the additional resistor 1 other than the LED bulbs of the light string is equal to 30V (i.e., 120V−90V) divided by 0.02 A according to Ohm's Law. Hence, the resistance (R) of the resistor 1 is 1,500 Ω. DC 30V is applied to the resistor 1. Also, the resistor 1 is adapted to generate much less heat. Therefore, the light string is protected.
However, both the well known light strings suffer from a number of disadvantages. In detail, each resistor is required to dispose in the light string by soldering which may pollute the environment if sufficient care is not taken. LED bulbs may be damaged due to high temperature and electrostatic discharge (ESD) while soldering. Soldering is a manual, laborious task such that these light strings cannot be mass produced easily. Heat generated by the resistor may damage the adjacent LED bulb and even cause a fire hazard. Moreover, the manufacturing cost is very high.
There have been numerous suggestions in prior patents for light string. For example, U.S. Pat. No. 6,344,716 discloses a Christmas light string. Thus, continuing improvements in the exploitation of light string employing LED bulbs are constantly being sought.